The “hydrogen ion implantation method” utilizing hydrogen ion implantation is known as one of methods for preparing bonded-silicon on sapphire (B-SOS). Known is a method involving bonding a hydrogen ion-implanted silicon substrate which has been surface activated to a sapphire substrate by the bonding technique, and causing mechanical separation at the hydrogen ion-implanted interface using a suitable mechanical means such as driving in a wedge, injecting a water jet, or irradiating light to the interface, yielding a thinned silicon film.
As another approach, a method for preparing a support substrate having SOI structure, by bonding a SOI substrate including a silicon substrate as base substrate to a support substrate, grinding and etching the SOI substrate to remove the silicon substrate thereof was proposed. Development efforts are currently continued.
In this case, it is important to what extent the silicon substrate is ground or how to trim an outer peripheral portion of bonded substrate. In JP-A 2011-071487 (Patent Document 1), for example, the trimming technique is the key. As the timing of trimming, after a bonded wafer is ground, a periphery thereof is trimmed, and after trimming, both the peripheral portion and an active portion are treated by chemical etching. As opposed to the prior art where trimming precedes grinding for thinning, better results are allegedly obtained when grinding for thinning is followed by trimming.
Also, there are function-enhanced substrates including silicon-on-quartz (SOQ) which is utilized in optoelectronic applications due to high transparency, and silicon-on-sapphire (SOS) which is utilized in high-frequency applications due to tight insulation and good heat conduction. It is known that since these composite or hybrid substrates are obtained from a combination of silicon serving as a semiconductor layer with a material having a different coefficient of thermal expansion, their manufacture is difficult, when they are manufactured by bonding, because of the difference in coefficient of thermal expansion between two substrates.
The known methods for manufacturing SOI type hybrid substrates by bonding include the following.
(1) One method is a so-called “smart cut” method involving implanting hydrogen ions into a silicon wafer having an oxide film formed as by thermal oxidation, bonding the wafer to a support substrate, effecting consolidation heat treatment, and applying heat for thermal separation. The method is such that heat treatment at high temperature causes the implanted gas to form a microbubble layer in the bulk of substrate whereupon separation occurs by expansion of the bubble layer. High-temperature heat treatment is thus essential, making it difficult to apply the method to substrates having different coefficients of thermal expansion.
(2) Another method is a SiGen method involving implanting hydrogen ions into a silicon wafer having an oxide film formed as by thermal oxidation, bonding the wafer to a support substrate, effecting consolidation heat treatment, and effecting mechanical separation. This method does not need the action of collection and expansion of the bubble layer in the bulk, high-temperature heat treatment is unnecessary, and the bond strength is previously increased by activating the bonding interface with plasma or the like, allowing heat treatment at lower temperature. The stack is not exposed to high temperature as in the smart cut method. However, mechanical separation cannot avoid that stresses are locally applied to a portion of the bonded substrate, with the drawback that the silicon thin film in that portion is prone to be defective. If the temperature of heat treatment is elevated to increase the bond strength for preventing the drawback, the problem of coefficient of thermal expansion arises like the smart cut method. Notably, the portion where stresses are locally applied includes a periphery where the bonding interface in bonded substrate breaks off and the terminus of separation, the circumference of silicon thin film becomes irregular or includes fine pits (microscopic film thickness variations).
While the above two methods rely on the ion implantation/peeling procedure of implanting hydrogen ions and effecting separation (or exfoliation) at the defective layer caused by hydrogen ions, problems may arise including defects spreading from the defective layer and more defects resulting from diffusion of hydrogen gas species. In particular, high-temperature treatment such as thermal oxidation treatment can generate defects.
(3) In contrast to the above, the method not resorting to the ion implantation/peeling procedure is a method involving bonding a silicon wafer having an oxide film formed by thermal oxidation to a support substrate, effecting consolidation heat treatment, grinding or etching the back surface of the silicon substrate for thinning the silicon substrate, thereby finishing a silicon thin film of the desired thickness. When the thickness of the finished silicon thin film is thin, the amount of material removed (working amount) is increased. Thus this method fails to reduce the variation of in-plane thickness. Studies are made on the technique of increasing accuracy by implanting oxygen ions to form a stop layer against grinding or etching. This method suffers from the problem of film thickness distribution and the problem that a peripheral portion is not neat since an unbonded portion at the periphery can be chipped away during the grinding step or left behind. Sometimes the step of trimming the periphery prior to or during the grinding step must be added, making the method cumbersome.